1) Field of the Invention
The present invention relates to a method of depositing a silicon dioxide film and, more particularly, it relates to an improved method of depositing a silicon dioxide film on the surface of a substrate by bringing the substrate into contact with an aqueous solution of hydrosilicofluoric acid supersaturated with silicon dioxide.
2) Description of the Prior Art
It is widely carried out these days to coat the surfaces of various materials with silicon dioxide films. For example, it has been practiced for many years to reduce the reflection of a surface, e.g., a glass surface by applying an alternating multi-layered coating of titanium oxide films and silicon dioxide films on the surface. It is also practiced extensively to form a silicon dioxide coating as a protecting film on the surface of a metal or alloy. Besides, when an alkali metal-containing glass such as soda lime glass or borosilicate glass is used as liquid crystal display panel or solar battery substrate glass, the surface of such a glass is coated with silicon dioxide in order to avoid the elution of its alkali metal component or components. Prevention of the elution of the alkali metal component or components from glass has become a technique particularly indispensable for maintaining long a service life of liquid crystal display devices or solar batteries.
A variety of techniques has heretofore been employed to deposit silicon dioxide films on glass surfaces, including the vacuum deposition, sputtering, CVD and dip coating (dipping) techniques. These conventional techniques are, however, accompanied by such drawbacks that the formation of silicon dioxide films is costly as these conventional techniques require expensive facilities and accessories and, besides, they can be applied to small substrates only.
In contrast to the above techniques of depositing the silicon dioxide films, there is known a method, which can be effected with simple facilities and is capable of depositing the silicon dioxide film on a large substrate, and in which the silicon dioxide film is formed by deposition on the surface of a substrate by bringing the substrate into contact with a treating solution prepared by adding boric acid to an aqueous solution of hydrosilicofluoric acid substantially saturated with silicon dioxide. (See, for example, Japanese Patent Application Laid-open No.60-33233). This method will hereinafter be referred to as "liquid phase deposition method".
The principle of depositing a silicon dioxide film by the liquid phase deposition method is that, when the treating solution of hydrosilicofluoric acid solution substantially saturated with silicon dioxide is in the following equilibrium state: EQU H.sub.2 SiF.sub.6 +2H.sub.2 O.revreaction.SiO.sub.2 +6HF (1)
and boric acid is added to this treating solution, hydroborofluoric acid is then produced from boric acid and hydrofluoric acid in accordance with the following reaction: EQU H.sub.3 BO.sub.3 +4HF.fwdarw.HBF.sub.4 +3H.sub.2 O (2)
and, as a result of the consumption of hydrofluoric acid, the equilibrium in formula (1) proceeds to the right-hand side, and the amount of SiO.sub.2 in the treating solution is increased to a supersaturation level, followed by the deposition of a film of SiO.sub.2 on the surface of the substrate.
The above deposition method has advantages that the formation of the film is possible at a low temperature, and that it is possible to form films on substrates of any materials and any shapes, but it has disadvantage, on the other hand, that a complicated process is required for detoxifying treatment of the effluent of the used treating solution to be discarded.
In a fluorine-containing effluent, the fluorine is generally separated and removed as a CaF.sub.2 precipitate formed by the addition of Ca(OH).sub.2. However, the fluorine concentration in the effluent produced in the conventional liquid phase deposition method cannot readily be decreased by single addition of Ca(OH).sub.2, and it is therefore necessary to repeat many times the cylcle of the addition of Ca(OH).sub.2, precipitation and separation.